Prior to and shortly after World War II, most commercial and residential floor tile installations utilized "mud setting" beds. These beds were composed of a lean mixture of sand and cement, placed fairly dry and generally not bonded to the floor base surface. Typically the mud setting bed was separated from the base of 15 pound roofing felt or the like. Tiles were fairly thick, e.g. about 3/4" to 2" thick, and the mud beds were generally in the range of about 1-1/4" to 1-1/2" thick. The same basic systems were used for terrazzo flooring.
Since the flooring systems were not bonded to the base, the base was free to move laterally with respect to the rest of the system. While this created some problems, it also offered the significant advantage that both the tile and the base (when a concrete base was used, which was typical) were protected from cracking. Shear forces caused by horizontal movement of the base were not transferred to the top finished surface. In addition, the very thickness of the system permitted a transfer of impact loads to dissipate to minimal levels prior to reaching the base level.
While flooring systems as above described were long lived and protected tiles from cracking, they were costly and heavy, and tile installations of this type were not easily coordinated with installations of carpet or vinyl floor covering.
Beginning in the early 1950's, the thick tile floor systems described above gave way to thin set systems, utilizing much thinner tiles, rarely over 1/2" thick, which frequently were direct-bonded to a concrete or wood substrate. Flooring systems of this type are less costly, lighter, and are more easily coordinated with installations of carpet or vinyl flooring. However, direct bonding of hard surface materials to a hard solid substrate, either concrete or wood, has caused problems. Concrete shrinks. Wood expands and contracts. These dimensional changes in the substrate transmit forces to the surface finish, whether tile or terrazzo, causing the direct bonded tile or terrazzo to crack.
The problem of cracking can be solved relatively easily when a wooden base or substrate is used. One simply nails expanded metal lath to the wooden base. Installations of this type have been in use for some 20 years, and give fairly good protection against cracking to the surface finish material. This solution is not readily applied to systems having a concrete base, however. It is difficult and expensive to "nail", i.e. mechanically affix lath to concrete. Various solutions to the cracking problem have been proposed. Basically, these involve the placement of a thin membrane between the concrete base and the tile. There are two basic types of such membranes: those which are solid when applied, and those which are liquid when applied. The former emanate primarily from the roofing industry, and comprise a soft plastic, in some cases elastomeric, material in thin sheet form. The liquid applied membranes dry to a soft solid. These membranes will absorb the horizontal movement of concrete and tile. However, they dramatically lower impact resistance. As a result, tiles and terrazzo are easily broken by workers' tools, wheel loads, or any other localized high stress. In short, significant tile cracking problems remain.